A mobile terminal in a prior art mobile communications system is so constructed as to transmit data to a base station as soon as it receives the data, but as not to transmit the data after carrying out scheduling of the transmission timing of the data.
However, the power of transmission of data in the mobile terminal increases and the interference level of a base station increases with increase in the rate of transmission of data. Therefore, the scheduler of a base station is requested to prevent the increase in the interference level so that the interference level does not exceed a certain level by controlling the transmission timing or the like of each mobile terminal, thereby increasing the throughput.
The scheduler of a base station can also shift the time when the amount of transmission data reaches its peak by controlling the transmission timing or the like of each mobile terminal in consideration of the interference level.
Conventionally, the interference level of a base station cannot be controlled, and the transmission rate is limited while there is provided a certain level of margin for the interference level in advance. Therefore, if the interference level of a base station can be controlled, the margin is reduced so that the peak rate of transmission of data can be increased.
In uplink transmission of data, data transmitted from mobile terminal can reach two or more base stations, and another base station other than a base station which takes charge of the scheduling is able to receive the data.
A process of receiving data transmitted from mobile terminal by means of two or more base stations so as to improve the reception quality is called macro diversity, and, in a case in which two or more base stations which receive data transmitted from one mobile terminal exist, a base station which takes charge of the scheduling process for the mobile terminal is called a primary (Primary) base station or a serving (Serving) base station.
A base station which did not take charge of the scheduling process and which simply receives data transmitted from mobile terminal is called a non-serving base station, and a set of two or more base stations which communicate with one certain mobile terminal is called an active set.
Even a non-serving base station mounts a scheduler therein because it needs to carry out the scheduling process for other mobile terminal, and serving base stations (serving NodeBs) and non-serving base stations are distinguished from each other according to whether they take charge of scheduling for each mobile terminal.
Conventionally, also in uplink transmission of data, while macro diversity is used during a soft handover, all base stations which are included in the active set are allowed to receive data via a radio link during the soft handover.
However, in high-speed packet communications in which a scheduler is introduced, while the aim is to cover a high error rate with control of retransmission of data to base stations by lowering the power of transmission of data, mobile terminal increases its power of transmission of data excessively to transmit data so that all base stations can receive the data.
In contrast, in a case in which only a serving base station which serves as the scheduler is made to receive data from mobile terminal, if the quality of a transmission line via which the data is transmitted is changed and the transmission line quality degrades, many processes of retransmitting data will occur and the throughput will degrade.
Therefore, in high-speed packet communications in which a scheduler is introduced, it is desirable that neither all base stations included in the active set nor only the serving base station receives data from the mobile terminal, but two or more base stations receive the data from the mobile terminal in order to ensure the link quality during a soft handover.
A soft handover technology without using a scheduler has been known conventionally. For example, a method of optimizing the number of base stations included in the active set is disclosed by patent references 1, 3, and 4, and an improvement in the channel status is disclosed by patent reference 2.
Patent reference 1 discloses a method of mobile terminal carrying out wireless communications with base stations, measuring the intensities of signals from them, RF performance, or the like, and adjusting the number of base stations in the active set with two thresholds (if there exists one or two base stations having a signal intensity or the like greater than the first threshold, the mobile terminal selects a base station to be included in the active set, and, if there exists one or two base stations having a signal intensity or the like greater than the second threshold, it selects two base stations from the active set).
However, patent reference 1 only discloses a technique for only measuring signal intensities, RF performance, or the like for the purpose of economization of radio resources, and for limiting the number of base stations in the active set.
Patent reference 2 discloses a method of carrying out an intra cell handover when the number of times that data is retransmitted becomes equal to or larger than a fixed number of times, thereby improving the channel status and continuously carrying out data communications.
However, in accordance with this method disclosed by patent reference 2, an intra cell handover is carried out only for the purpose of an improvement in forced disconnection.
Patent reference 3 discloses a method of using macro diversity when the reception state of a radio unit is bad, and not using macro diversity when the reception state of the radio unit is good.
However, the method in accordance with patent reference 3 cannot adjust the interference level of uplink, and therefore cannot provide the advantage of the macro diversity.
Patent reference 4 discloses a method of removing a base station in an overloaded state from an active set by lowering the input/output reference of the base station in the overloaded state, to reduce the size of the active set.
However, the method in accordance with patent reference 4 is a technique applicable only to downlink data transmission, and cannot be applied to uplink data transmission.
In nonpatent reference 1, there is a reference to an E-DCH (Enhanced Dedicated CHannel) active set (E-DCH active set) via which high-speed packet communications are carried out using a scheduler, and a method of selecting a base station is disclosed as a reference for addition of a base station to the E-DCH active set. More specifically, by means of the method, a base station is selected on the basis of DPCCH SIR (Signal to Interference Ratio) or E-DPCCH BLER (BLock Error Rate) measured by a base station, which indicates the quality of the uplink transmission line.
Although the quality of the uplink transmission line is important when selecting a base station to be included in the E-DCH active set, the interference level of each base station, rather than the quality of the uplink transmission line, needs to be taken into consideration.
However, the method of selecting a base station in accordance with nonpatent reference 1 does not take into consideration a margin for the interference level of a non-serving base station.
Furthermore, in accordance with the prior art method, a base station is selected without taking the load caused by signaling into consideration. Although when actually selecting a base station, it can be assumed that an apparatus other than base stations can carry out the selection, nonpatent reference 1 discloses neither any technique for making a mobile terminal select a base station, nor any technique for making a radio network controller select a base station. Furthermore, nonpatent reference 1 does not make a study of concrete processes.
[Patent reference 1] JP, 2002-95031,A
[Patent reference 2] JP, 2002-77982,A
[Patent reference 3] JP, 2001-16633,A
[Patent reference 4] JP, 2001-197536,A
[Nonpatent reference 1] 3GPP RAN1 document R2-042357
A problem with prior art mobile communications systems which are constructed as mentioned above is that while a base station which takes charge of scheduling can limit the power of transmission of data from mobile terminal in consideration of the interference level of the base station, a base station which does not take charge of the scheduling cannot limit the power of transmission of data transmitted from the mobile terminal even if the interference level of the base station exceeds a maximum allowable level and therefore the transmission quality degrades.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a mobile communications system which can suppress degradation in the transmission quality by enabling a base station whose interference level exceeds a maximum allowable level to control the power of transmission of data transmitted from mobile terminal, and the mobile terminal.